1. Field of the Invention
The present invention relates to a method to confine plutonium in apatitic ceramics and the products obtained using said process.
More specifically, it relates to a method to fix plutonium in atomic form in a ceramic mineral phase providing a durable confinement material.
With the end of the Cold War, the signing of the Nuclear Weapon Non-proliferation Treaty and the discontinuation of nuclear testing, some countries are in possession of a large quantity of plutonium which requires storage.
Plutonium is a radioactive element with a long life (Pu-239: T=24390 years), hence the need to develop extremely stable conditioning matrices. To avoid criticality problems, the matrix must prevent concentration in the form of clusters.
The purpose of this invention is to provide a method for preparing a confinement matrix intended to store plutonium, the durability and long-term stability of which are confirmed by the existence of natural britholites (OKLO xe2x80x9cnaturalxe2x80x9d reactor) containing long-life actinides.
2. State of the Related Art
It has already been envisaged to condition radioactive waste in silicate apatites, as described in the document WO-A-95/02886 [1]. In this document; the radioactive waste conditioned may be lanthanides and actinides.
As indicated in this document, the use of phosphosilicate apatite is very advantageous since apatites have the following remarkable properties:
these structures are very chemically and thermally stable in the natural environment. In addition, natural silicate apatites containing actinides have existed for hundreds of millions of years.
Apatites have a very low solubility in water; in addition, their solubility decreases as the temperature increases, which is a positive point for actinide conditioning since their high activity involves an increase in temperature in the matrix containing them.
Apatitic structures are capable of withstanding radioactivity since the irradiation damage incurred is unstable at temperatures above 60xc2x0 C. Apatites have the property of restructuring at temperatures above 60xc2x0 C.
Apatitic structures have the very advantageous property of being able to incorporate numerous metallic elements, particularly actinides and lanthanides, in their structure.
Natural apatites may be represented in the following formula:
Ca10(PO4)6F2
which corresponds to the structure of fluoroapatite. However, in this structure, numerous substitutions may be made, particularly by replacing Ca2+ cations by various metallic elements such as lanthanides, actinides and alkaline metals, substituting PO43xe2x88x92ions by other anions such as SiO44xe2x88x92, and also substituting Fxe2x88x92anions by S2xe2x88x92, Clxe2x88x92, Brxe2x88x92, Ixe2x88x92 or OHxe2x88x92 anions.
The formulations used in this document take the following form:
Mxe2x88x92Cax LnyAz (PO4)u(SiO4)6xe2x88x92uX
where
M=an alkaline metal,
Ln=Y or a lanthanide,
A=an actinide
X=S2xe2x88x92, 2Fxe2x88x92, 2Clxe2x88x92, 2OHxe2x88x92, 2Brxe2x88x92 or 2Ixe2x88x92.
0xe2x89xa6txe2x89xa63,
0 less than u less than 6,
0 less than xxe2x89xa610,
0xe2x89xa6yxe2x89xa610,
0 less than zxe2x89xa67, and
y+z greater than 0
These apatite formulations may comprise many SiO44 ions, therefore few PO43xe2x88x92ions, and for this reason show poorer xe2x80x9cself-curingxe2x80x9d against irradiation from xcex1 radiation emitted by the Pu. Indeed, phosphate tetrahedrons are more rigid than tetrahedrons formed by silicates.
In addition, the method to manufacture said apatites requires the use of excess CaF2, for example, a 10% excess with reference to the stoichiometric quantity required to obtain the fluorinated apatite, and several calcination steps with intermediate grinding to obtain the fluorinated apatite.
When a dense ceramic is required, the powder obtained is subjected after the calcination steps to sintering, for example hot compression sintering. Therefore, this method requires numerous calcination steps followed by sintering to obtain a dense ceramic.
The present invention specifically relates to new phosphosilicate apatite formulations, suitable for plutonium confinement, and a method to prepare said apatites in dense ceramic form which is easier to use.
In this way, the invention relates to a plutonium confinement block comprising a phosphosilicate apatite matrix containing the plutonium to be confined and any radioactive lanthanides, the plutonium and lanthanides being included in the chemical structure of the phosphosilicate apatite and said apatite complying with the following formula:
MtCaxLnyHfwPuzxe2x88x92w(PO4)6xe2x88x92u(SiO4)uF2xe2x80x83xe2x80x83I
wherein:
M represents at least one alkaline metal;
Ln represents at least one cation selected from the lanthanides, and
t, x, y, z, w and u are such that:
0xe2x89xa6txe2x89xa61,
8xe2x89xa6xxe2x89xa610,
0xe2x89xa6yxe2x89xa61,
0 less than zxe2x89xa60.5,
0xe2x89xa6w less than z, and
0 less than uxe2x89xa6y+2z,
and the total number of positive charges provided by the M, Ca, Ln, Hf and Pu cations are equal to (20+u)
According to the invention, in the phosphosilicate apatite in compliance with formula I given above, the quantities of plutonium, and Ln, Hf and M cations may be varied within relatively small limits with reference to the calcium content x of the apatite, which is 8 to 10.
Similarly, on the tetrahedral sites (PO4 and SiO4), it is possible to vary the SiO4/PO4 ratio. Each substitution has an effect on the physico-chemical properties of the conditioning matrix and it is the specific choice of formulation that gives the matrix specific physico-chemical properties. The SiO4/PO4 ratio particularly controls the leaching resistance and the irradiation strength. Therefore, it is necessary to adjust said ratio with reference to the quantity of plutonium introduced. For this reason, according to the invention, the quantity of SiO4 cannot exceed the value y+2z, which corresponds to the quantities of Ln and Pu cations present in the phosphosilicate apatite.
In the formula I given above, the total number of negative charges is supplied by the PO43xe2x88x92, SiO44xe2x88x92 and Fxe2x88x92 anions. Said charges are balanced by the M monovalent cation, the Ca divalent cation and the trivalent and tetravalent cations which may be lanthanides, Hf and Pu.
To ensure that the charges are balanced, the quantities t, x, y, w and z are selected according to the quantity (6xe2x88x92u) of the PO4xe2x88x92 anions and the valence of the Ln, Hf and Pu cations present to obtain neutrality. For this reason, said quantities may or may not be integers.
The alkaline metal used in said apatite may be any alkaline metal. Na and K are selected advantageously for their easy use. The lanthanides may be any lanthanide from La to Gd. They may be radioactive or not. The choice of Ln cation is important since it makes it possible to substitute the apatite with a neutrophage product such as Gd, to reduce criticality risks.
According to the invention, the confinement block may be composed exclusively of phosphosilicate apatite containing Pu, which preferentially, contains no alkaline metal. However, it is also possible to have, around this apatite matrix containing the plutonium to be conditioned, at least one non-active silicate apatite layer and, if applicable, other layers of silicate apatite or not, of different compositions, so as to form appropriate successive barriers between the plutonium and the environment.
Preferentially, the phosphosilicate apatites of the block and/or the most inner layers directly in contact with the plutonium, do not contain any alkaline metal for improved resistance to irradiation damage, and the most outer layers may be selected to withstand attacks from the outer environment. It is possible to use, for one or more layers, the fluoroapatite composition Ca10(PO4)6F2 which is particularly resistant.
In addition, the layers are chosen so as to improve the overall mechanical properties.
According to a first embodiment of the invention, the phosphosilicate apatite formulation is such that the number of silicates is equal to the number of Pu in the crystallographic mesh, the apatite comprising no Ln cations. In this case, the phosphosilicate apatite may comply with the following formula:
NazCa10xe2x88x922zPuz(PO4)6xe2x88x92z(SiO4)zF2xe2x80x83xe2x80x83II
wherein 0 less than zxe2x89xa60.5
An example of such a phosphosilicate apatite may comply with the formula:
Na0.45Ca9.1Pu0.45(PO4)5.55(SiO4)0.45F2xe2x80x83xe2x80x83III.
According to a second embodiment of the invention, a phosphosilicate apatite wherein the PO4/SiO4 ratio is equal to 5 is chosen, and the Ca and Ln cation contents are chosen as a function of the quantity of Pu introduced to meet the phosphosilicate apatite neutrality condition.
In said second embodiment of the invention, the phosphosilicate apatite may comply with the formula:
Ca9+zGd1xe2x88x922zHfwPuzxe2x88x92w(PO4)5(SiO4)1F2xe2x80x83xe2x80x83IV
where 0 less than zxe2x89xa60.5 and 0xe2x89xa6w less than z.
An example of such a phosphosilicate apatite may comply with the formula:
Ca9.46Gd0.08Pu0.46(PO4)5SiO4F2xe2x80x83xe2x80x83V.
According to a third embodiment of the invention, the phosphosilicate apatite""s formulation accounts for criticality criteria which may be controlled by the quantity of an element with a high effective neutron capture cross-section such as gadolinium. In this case, the proportion of Gd introduced is adjusted to the criticality risk and not chosen as a function of the quantity of plutonium, as in the second embodiment of the invention. This particular case of introducing a trivalent element associated with the plutonium will be of particular interest for plutonium of military origin since said plutonium is in the presence of gadolinium. In this case, the phosphosilicate apatite may comply with the formula:
Ca10xe2x88x92yxe2x88x92zGdyHfwPuzxe2x88x92w(PO4)6xe2x88x922zxe2x88x92y(SiO4)y+2z F2xe2x80x83xe2x80x83VI
where 0 less than yxe2x89xa61, 0 less than zxe2x89xa60.5 and 0xe2x89xa6w less than z.
An example of such a phosphosilicate apatite may comply with the formula:
Ca9.04Gd0.48PU0.48(PO4)4.56(SiO4)1.44F2xe2x80x83xe2x80x83VII.
According to a fourth embodiment of the invention, the phosphosilicate apatite comprises only plutonium and calcium, the Pu substituting the calcium. In this case, the phosphosilicate apatite may comply with the following formula:
Ca10xe2x88x92zPuz(PO4)6xe2x88x922z(SiO4)2zF2xe2x80x83xe2x80x83VIII
where 0 less than zxe2x89xa60.5.
An example of such an apatite may comply with the formula:
Ca9.55Pu0.45(PO4)5.1(SiO4)0.9F2xe2x80x83xe2x80x83IX.
The formula I phosphosilicate apatites used in the invention may be prepared using powder from constituents in oxide, phosphate, carbonate, fluoride, halide, sulphide, hydroxide, or silicate form.
In addition, the invention also relates to a method to prepare a phosphosilicate apatite complying with the following formula:
MtCaxLnyHfwPuzxe2x88x92w(PO4)6xe2x88x92u(SiO4)uF2xe2x80x83xe2x80x83I
wherein:
M represents an alkaline metal;
Ln represents at least one cation selected from the lanthanides, and
t, x, y, z, w and u are such that:
0xe2x89xa6txe2x89xa61,
8xe2x89xa6xxe2x89xa610,
0xe2x89xa6yxe2x89xa61,
0xe2x89xa6w less than z,
0 less than zxe2x89xa60.5, and
0 less than uxe2x89xa6y+2z,
and the total number of positive charges provided by the Na, Ca, Ln, Hf and Pu cations are equal to (20+u), from a mixture of powders comprising the following reagents: plutonium dioxide, calcium pyrophosphate, compounds of the different constituents of the apatite to be prepared and at least one fluorinated reagent, according to which the following steps are carried out:
1) preparation of a first mixture of powders comprising all the reagents except for the fluorinated reagent(s);
2) addition of the first mixture of fluorinated reagent(s) to obtain a final mixture of all the reagents;
3) grinding of the final mixture to a particle size of less than 50 xcexcm;
4) sintering-reaction of the final mixture, at a temperature of 1100 to 1600xc2x0 C., in a neutral or reducing atmosphere, with application of pressure before or during the sintering.
To implement the method according to the invention, in step 2, quantities of fluorinated reagent(s) corresponding to the stoichiometric proportions required to obtain the formula I phosphosilicate apatite are advantageously used.
In this way, with the method according to the invention, particularly due to the preparation of the powder mixture in two steps, it is possible to obtain a total incorporation of the fluorine during the sintering-reaction step and thus obtain a homogeneous dense ceramic in which the plutonium is immobilised.
According to the invention, xcex2-form calcium pyrophosphate Ca2P2o7 obtained by calcining anhydrous or dihydrate calcium hydrogen phosphate (CaHPO4 or CaHPO4.2H2O) is used, at approximately 1000xc2x0 C., for 1 to 2 hours, since it is easier to check its purity in this form.
The plutonium is introduced in the form of plutonium oxide which is ground alone to obtain a powder with an average particle size of 2 to 50 xcexcm, e.g. a particle size of approximately 10 xcexcm. This very fine grinding of the plutonium oxide makes it possible to obtain in the final ceramic the most homogeneous distribution of plutonium possible. In addition, the ground quantities prevent criticality from being reached.
After these two powders have been prepared, the reagents are weighed in stoichiometric proportions and all the reagents are mixed with the PuO2 powder by performing steps 1 and 2 of the method according to the invention, to introduce the fluorinated reagent(s) last.
The first step may be performed by mixing all the powder reagents except for the fluorinated reagents such as calcium, gadolinium and/or plutonium fluorides, with the PuO2 powder, in a liquid used as a lubricant and homogeniser such as water, an alcohol or acetone. Other liquids may be suitable provided that they can evaporate and do not leave any residue. The quantity of liquid is such that the mixture of liquids is covered well. Mixing is performed until the liquid, e.g. acetone, has evaporated. Then the residue is placed to dry in an oven at approximately 100xc2x0 C. to evaporate the acetone completely. The residue is then ground to obtain a powder with a good homogeneity and a particle size less than 50 xcexcm.
When the mixture of powders added to PuO2 comprises calcium carbonate, the first mixture obtained in the first step is subjected to a heat treatment to break down the calcium carbonate before adding the fluorinated reagents. Said heat treatment may consist of heating for 1 to 2 hours at 900xc2x0 C.
In the second step, the pulverulent fluorinated reagents are added to the powder which has undergone the heat treatment. For this, both powders are mixed, e.g. in acetone (sufficient quantity to cover the mixture) until the acetone evaporates. Subsequently, the reaction mixture is placed in an oven to eliminate the residual acetone at a temperature of not more than 120xc2x0 C. to prevent any risk of volatility of the fluorinated products.
Step 3) of the method according to the invention, which consists of grinding the powder mixture to particle size of less than 50 xcexcm, is then carried out. Said grinding may be performed in the presence of distilled water by adding, for example, 50% by weight of distilled water to the powder mixture comprising all the reagents. The mixture is then ground in jars in ZrO2 for approximately twenty minutes at a speed of approximately 1100 rpm. After drying in an oven for approximately 2 hours at around one hundred degrees, the powder is sieved to ensure that no particles of over 50 xcexcm are retained.
The fourth sintering-reaction step may be performed in two different ways depending on whether pressure is applied before or during the sintering.
According to a first embodiment of said sintering reaction step, the following steps are performed:
a) compressing the mixture of powders in a mould under pressure from 100 to 500 MPa (1000 to 5000 bar), and
b) calcining the compressed product, in a neutral or reducing atmosphere, at atmospheric pressure, at a temperature from 1500 to 1600xc2x0 C.
In this case, the reagent powder obtained in step 3) is compacted, e.g. in a cylindrical mould of a diameter of approximately ten millimetres, with pressures ranging from 100 to 500 MPa (1000 to 5000 bar).
This compacting is required to improve the reactivity of the powder by placing the particles in contact. Preferentially, the pressure is applied slowly so as not to trap the air and a stabilised phase of 15 minutes, when ⅔ of the final pressure has been reached, may be envisaged. The pellets obtained after compacting are calcined in a tubular furnace enabling operation in a neutral (e.g. nitrogen) or reducing (e.g. a mixture of argon and 5% hydrogen) medium. The calcination is performed at a high temperature (between 1500xc2x0 C. and 1600xc2x0 C.) for times ranging from 2 to 20 hours depending on the quantity of plutonium to be incorporated (1 to 10% by weight). The temperature rise and fall rate may vary by 5 to 50xc2x0 C. per minute depending on the furnace""s capacities. The apatite synthesis and sintering are performed in a single step, which limits the volatility of the fluorine, and the high temperatures used make it possible to obtain very homogeneous ceramics.
In this way, said sintering-reaction step makes it possible to obtain a dense ceramic without the need to perform several calcination cycles separated by grinding steps as in document [1], and without ending the treatment with pressurised sintering.
According to a second embodiment of the sintering-reaction step 4), the mixture of powders is sintered in a neutral or reducing atmosphere, at a pressure of 10 to 25 MPa (100 to 250 bar) at a temperature of 1100 to 1500xc2x0 C.
Said sintering-reaction may be completed if required to improve the homogeneity of the matrix further, by an additional treatment comprising the following steps:
c) grinding of the sintered product obtained,
d) compacting of the ground product at a pressure of 100 to 500 MPa (1000 to 5000 bar), and
e) annealing heat treatment of the compacted product, at atmospheric pressure, in a neutral or reducing atmosphere, at a temperature of 1200 to 1600xc2x0 C.
For example, these different steps may be performed as follows.
The reagent powder mixture obtained in step 4) is compacted in a graphite mould at ambient temperature at a pressure of 10 to 30 MPa (100 to 300 bar).
A first temperature stage (between 600xc2x0 and 800xc2x0 C.) is reached at a rate of 5 to 50xc2x0 C./minute without pressure to prevent air trapping. The pressure is only applied at the end of the fifteen-minute stage and up to the end of the final stage (between 1100xc2x0 C. and 1500xc2x0 C.). The duration of the stage, during which the pressure is applied, depends on the temperature (between 1 hour and 12 hours) and the quantity of plutonium (1 to 10% by weight).
If necessary, the pellet obtained may then be ground and reformed by compacting at a pressure of 100 to 500 MPa (1000 to 5000 bar). The pellet obtained undergoes a further calcination or annealing to improve the distribution of Pu in the matrix. The annealing, at atmospheric pressure, is performed at a temperature of 1200xc2x0 C. to 1600xc2x0 C., in a neutral or reducing atmosphere, for 2 to 20 hours, depending on the temperature. The higher the sintering-reaction temperature, the shorter the annealing time required to obtain a homogeneous apatitic ceramic.